Low-noise detection of infrared radiation in the mid-IR range is challenging due to the thermal background radiation. The most widely used infrared spectrometer is the Fourier Transform Infrared (FTIR) spectrometer. Common FTIR spectrometers must scan a reference mirror with very high precision on a centimetre scale, requiring an extremely high precision mechanical system, with associated high costs, non-instantaneous measurements and generally a low tolerance for vibrations. Furthermore, detection of the radiation is commonly performed with mid-IR detectors.
It has been shown in “Room-temperature mid-infrared single-photon spectral imaging” (Jeppe Seidelin Dam, et al., Nature Photonics 6, pp. 788-793, 2012) how the angular dependent (non-collinear) phase-matching can be used to upconvert different spectral components in different angular directions. This, however, also implies that the upconverted light becomes a substantially concentric pattern with radii as a function of wavelength, i.e. each constituent frequency will emerge with a different angle, thus producing an effect similar to dispersion in a prism or diffraction by a grating. In the above reference, this is actively used as an approach to spectroscopy by detecting the upconverted light in the Fourier plane. The spectral resolution in this angular wavelength dependence is, however, relatively low, which inflicts on the resolution of the spectral measurements.
A mid-IR spectral measurement based on frequency up-conversion was described in “High-resolution mid-IR spectrometer based on frequency upconversion”, Qi Hu, et al., Optics Letters, 37(24), pp. 5232-5234, 2012. Upconversion spectral measurements are based on shifting the spectrum from the mid-IR region to the near visible spectral region where detectors are better developed and there are less thermal noise. The frequency shift is obtained by sum frequency mixing with a laser, resulting in a simple shift of the frequency while maintaining the spectral content for subsequent detection. In this reference, a wavelength range from about 2.89 μm-3.00 μm was up-converted using three different temperatures of the nonlinear crystal to phase-match different wavelength ranges. A drawback of temperature tuning the nonlinear crystal is that slow temperature changes must be used to avoid damage to coatings on end faces of the nonlinear crystal. Thus, acquisition time of a full spectrum in the described spectral measurement will be on the order of minutes. The temperature tuning will typically also change the laser cavity performance in an intracavity setup and degrade the mixing laser field due to thermal expansion of the nonlinear crystal. Furthermore, a relatively narrow wavelength range was detected.
WO 2015/003721 discloses a multichannel infrared upconversion spectrometer with several channels, each channel configured to upconvert different wavelength ranges. By using two or more up-conversion channels, an extended input wavelength range may be accepted and converted for detection. The present solution is different in several ways. Firstly, it is able to cover a much larger spectral range in a single channel, eliminating the need for use of multiple channels. Secondly, here we place the detector array in an image space relative to the non-linear crystal, whereas that work is based on placing the detector in a Fourier plane and analysing the ring patterns, relying on the dispersion (phase match condition) of the non-linear crystal to resolve the spectral information.
U.S. Pat. No. 4,980,566 describes an ultrafast time-resolved spectrometer. Here, an IR pico/femtosecond pulse having interacted with a sample is overlapped with another pulse inside a nonlinear crystal to upconvert the IR pulse. Although a long crystal is used, the pulses and the arrangement are optimised to achieve a short (<1 mm) collinear phase matching. The upconverted IR pulse then enters a traditional spectrograph via an entrance slit.